I remeber seeing this in the cinema as a kid and enjoying it immensely. It was a huge hit in my class and we were all anticipating it on video and DVD.

I rewatched it recently, and to my delight, it's still as funny as it was when I was nine years old. It sticks to the spirit of the original cartoons, and respects them greatly. It's amusing, without ever becoming forced or over-stupified.
The CGI Scooby Doo himself is a nice mix between a real Great Dane and the cartoon original. All the characters stay in character, although I did notice a few minor changes, but it's no biggy.

If you're a fan of the original cartoons, or you're just looking for a fun time, Scooby Doo is the way to go.